1. Field of the Invention
The invention relates to a clock recovery circuit, and in particular to an EFM clock recovery circuit capable of automatically adjusting the frequency range of a voltage-controlled oscillator.
2. Description of the Related Art
In general, if an optical disc is rotated in a constant angle speed by an optical disk drive, the linear speeds at the inner and outer tracks of the optical disc are different. Under this condition, the frequencies of the EFM (Eight To Fourteen Modulation) signals at the different tracks read by the optical disk drive are not the same. In order to correctly read the information at different zones (tracks) of the optical disc, the optical disk drive has to adopt a clock recovery circuit (or a phase-locked loop circuit) to generate clocks in synchronization with the EFM signals.
FIG. 1 shows a typical EFM clock recovery circuit. The clock recovery circuit 100 includes a phase detector 101, a frequency detector 102, a charge pump 103 connected to the phase detector 101 and the frequency detector 102, a loop filter 104 connected to the charge pump 103, a voltage-controlled oscillator (hereinafter referred to as VCO) 107 for receiving the control voltage VT of the loop filter 104, and a frequency divider 106 for dividing the frequency of the clock FCO outputted from the VCO 107. Since the functions of the clock recovery circuit 100, phase detector 101, frequency detector 102, charge pump 103, loop filter 104, VCO 107 and frequency divider 106 are the same as those of a typical phase-locked loop (PLL) circuit, detailed descriptions thereof are omitted. In addition, the clock recovery circuit 100 further includes a digital-analog converter (hereinafter referred to as DAC) 105. The DAC 105 converts data outputted from the look-up table 105′ into an analog voltage RVT according to the data that is to be read by the optical disk drive and corresponds to the zone of the optical disc. Then, the analog voltage RVT is outputted to the VCO 107 so as to control the frequency range of the VCO 107. The look-up table 105′ is controlled by a control signal, thereby outputting data corresponding to the zone of the optical disc.
However, the clock recovery circuit 100 as shown in FIG. 1 has some drawbacks. First, if the pickup head of the optical disk drive moves to an incorrect zone, the DAC 105 cannot be updated to correct data. Thus, the VCO 107 may be saturated because the optical disk drive cannot read correct addresses. Under this condition, the optical disk drive has to be reset. Second, since the look-up table 105′ only records the relationship between the zones and frequencies, the tuning voltage VT varies with the speed variation of the spindle motor when the speed of the spindle motor of the optical disk drive becomes unstable. However, the output from the DAC 105 will never change and the VCO 107 will thus be saturated.